The invention relates to a method of folding pliable material in the form of assembled or single sheets of paper or similar relatively rigid material which may vary in quality and length, the folding taking place by means of folding rollers arranged approximately diametrically opposite to each other, substantially on either side of a feeding plane, starting from a position of the pliable material in which a part thereof lies in the feeding plane and a part is bent out of said plane and wherein an arcuate part of the portion of the pliable material bent out of the plane is bent further and is conducted to a folding/nipping area between the folding rollers where the folding rollers make the fold and wherein a trailing portion of the pliable material is fed in controlled fashion from the feeding plane.
Such a method is known from Dutch laid-open patent application 8702456. In this method the curve of the portion of the pliable material that is bent out of the feeding plane is rendered more acute by pressing a so-called folding knife against it until the bent portion has a small radius and is pressed between the folding rollers The portion pressed between the folding rollers and, subsequently, the remaining portions of the pliable material are conducted through the folding nip by the folding rollers, so that the fold is made. This folding method can be used very universally. The position of the fold can be accurately set relatively to the trailing edge of the pliable material and the position of the folding line can be readily changed by chosing a different length for the portion bent out of the feeding plane in the above mentioned starting position of the pliable material. A set of pliable material, comprising stacked sheets of different size, can be folded as a whole, and the trailing portion of the pliable material fed from the feeding plane, in which no fold is to be made, may comprise relatively rigid material, such as cardboard or plastic, in virtue of the fact that the trailing portion of the pliable material traverses a substantially straight path during folding.
A problem involved in such a method, however, is the magnitude of the force required to press the knife against the pliable material for the pliable material to be folded accurately. In particular this is the case when thicker stacks of sheets are being folded, consisting, for example, of eight sheets on top of each other. Further, the knife must be controlled with great precision and it forms a long lever whose free end exerts a pressure. Such great forces to be exerted by and on the knife and the accurate control of the knife require a robust construction of the knife and its suspension, and, on account of the great mass of the construction, a great power for driving the operations of the knife. As a result, this method can only be carried out by means of an expensive, powerful and heavily-built machine.
The invention aims to overcome the above mentioned problems while retaining the advantages of the known method referred to.